I read a while ago an article (I could not find it back, but here is a similar one) about how humans used to sleep, at least during long winter nights, in two periods separated by an interval of nocturnal activity (including but not restricted to sleep), idenified in particular by references to the "first sleep" and "second sleep" in the literature. It is supposed to have ended with the coming of electricity and nocturnal light.
The article "rang a bell" in my mind as I used to work or read between two periods of sleep during various periods of my life (I have memories of doing so while still living at my parent's, so before 21; while in grad school, so between 21 and 25; and during the first year of my postdoctoral visit to Vancouver) and to be heavily criticized about it (and more generally about my sleep hygiene such as going to bed abnormally early such as 20:30) by the first girlfriend with whom I shared an appartment, in Vancouver. I don't remember doing too much of night working since then.
Another "bell" rang in my mind this morning while doing a Russian exercise: in Spanish and, if I understood correctly, in Russian, one wishes to the other "good nightS", where night is in the plurial form:
"доброй ночи" in Russian and "buenas noches" in Spanish. Could it be a leftover from older times when people wished others two good nights of sleep? This does not occur in French nor in English : both "bonne nuit" and "good night" are singular forms. The difference of evolution might be that the difference in pronunciation (as opposed to writing) between the singular and plurial form is non existent in French and subtle in English, while it is stronger in Spanish (two words are in the plurial form) and even stronger in Russian.
Those who read this and speak other languages (e. g. Laura, Luca, Sylvain), is the usual expression to wish "a good night" im the evening singular or plurial? And how similar are the singular and plurial forms?